The present invention relates to a bus driver, particularly, to a bus driver capable of reducing a ringing produced in the output waveform thereof.
Bus drivers of this type are conventionally provided in each of a plurality of electronic circuit packages and each of the packages is electrically connected to a bus through the bus drivers.
FIG. 17 is a schematic diagram for showing the connection between a plurality of electronic circuit packages and a bus. In FIG. 17, a bus B comprises a plurality of connectors C1, . . . , and C7 and bus terminating resistors R1 and R2. Connectors C1, . . . , and C7 are connected to electronic circuit packages, such as LSI (Large Scale Integration) packages, PKG-1, . . . , and PKG-7, respectively. Each of the connectors C1, . . . , and C7 is connected to each of PKG-1, . . . , and PKG-7 through a stub. Bus terminating resistors R1 and R2 terminate both the ends of bus B. Each of PKG-1, . . . , and PKG-7 is provided with bus drivers.
As shown in FIG. 18, a conventional bus driver has an open-drain-type structure using a MOS (Metal Oxide Semiconductor) transistor 109. That is, a source terminal S of MOS transistor 109 is connected to the ground level and a drain terminal D thereof is connected to an output terminal T. When a signal to be output from an LSI package is applied to a gate terminal G of MOS transistor 109 through an input terminal I of the bus driver, MOS transistor 109 is turned on or off in accordance with the state of the signal. When MOS transistor 109 is turned on, output terminal T becomes ground-level. When MOS transistor 109 is turned off, output terminal T is brought into a floating state.
In FIGS. 17 and 18, output terminal T of bus driver 19 is connected to connectors C1, . . . , and C7.
An example of this type of bus driver is disclosed in Japanese Patent Application Laid-Open No. Hei 2-242313.
In the case of the bus B, it is assumed that the distance between connectors C1 and C2 is 1.4 in., the distance between connectors C2 and C3, 2.0 in., the distance between connectors C3 and C4, 1.8 in., the distance between connectors C4 and C5, 1.8 in., the distance between connectors C5 and C6, 2.0 in., and the distance between connectors C6 and C7, 1.0 in. Moreover, it is assumed that the length of a stub from a circuit element on each LSI package to a connector is 1.0 in. when each LSI package is connected to the connector. Furthermore, it is assumed that the resistances of terminating resistors R1 and R2 are both 56 and resistors R1 and R2 are connected to 1.5-V DC power supply VT. In this case, simulation waveforms at output terminals of bus drivers of LSI packages PKG-1, PKG-3, . . . , and PKG-7 obtained by an output waveform from the output terminal of the bus driver of LSI package PKG-3 connected to connector C3 are shown in FIG. 19.
Curve 73 is a waveform of the LSI package PKG-3, curve 74 is a waveform of the LSI package PKG-4, curve 71 is a waveform of the LSI package PKG-1, curve 75 is a waveform of the LSI package PKG-5, curve 76 is a waveform of LSI package PKG-6, and curve 77 is a waveform of LSI package PKG-7. In FIG. 19, x-axis shows voltage value "bolt" and y-axis shows time "nanosecond".
As shown in FIG. 19, a ringing is produced in each output waveform at the rise time of the output waveform of the bus driver of LSI package PKG-3, that is, when the MOS transistor in the bus driver changes from on to off states. Particularly, curve 75 which is the waveform of LSI package PKG-5 has a lot of ringings and it is hardly kept in a range.
Therefore, a conventional bus driver has a problem that a ringing occurs in an output waveform.
This may be caused by reflections at both ends of each line constituting a bus. The ringing waveform in curve 75 vertically vibrates abov of below 1.5 V which is a terminating potential and particularly, it may be lower than V1 which is approximately 1.0 V.
As described above, because a considerably large ringing occurs in the conventional bus driver, it is difficult to transmit a signal with a narrow pulse width or a high repetitive frequency. Therefore, there is a problem that it is difficult to use a conventional bus driver in data transfer over a high-speed bus.
An object of the present invention is therefore to provide a bus driver capable of reducing a ringing produced in the output waveform thereof.